Method and apparatus for optimized sampling of volatilizable target substances

ABSTRACT

An apparatus for capturing, from gases such as soil gas, target analytes. Target analytes may include emanations from explosive materials or from residues of explosive materials. The apparatus employs principles of sorption common to solid phase microextraction, and is best used in conjunction with analysis means such as a gas chromatograph. To sorb target analytes, the apparatus functions using various sorptive structures to capture target analyte. Depending upon the embodiment, those structures may include 1) a conventional solid-phase microextraction (SPME) fiber, 2) a SPME fiber suspended in a capillary tube (with means provided for moving gases through the capillary tube so that the gases come into close proximity to the suspended fiber), and 3) a capillary tube including an interior surface on which sorptive material (similar to that on the surface of a SPME fiber) is supported (along with means for moving gases through the capillary tube so that the gases come into close proximity to the sorptive material). In one disclosed embodiment, at least one such sorptive structure is associated with an enclosure including an opening in communication with the surface of a soil region potentially contaminated with buried explosive material such as unexploded ordnance. Emanations from explosive materials can pass into and accumulate in the enclosure where they are sorbed by the sorptive structures. Also disclosed is the use of heating means such as microwave horns to drive target analytes into the soil gas from solid and liquid phase components of the soil.

This invention was made with Government support under ContractDE-AC0494AL85000 awarded by the U.S. Department of Energy. TheGovernment has certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention (Technical Field)

This invention relates generally to the field of sampling targetanalytes in which the target analytes, themselves, are in vapor phase.More specifically, the invention relates to using solid-phasemicroextraction in sampling of target analytes present in lowconcentration in a carrier gas, especially where the target analytes,such as emanations from explosive materials, are present in a matrix,such as soil, which comprises gas, liquid and solid components.

2. Background Art

The world faces the growing problem of locating and remediating hiddenexplosives hazards such as buried land mines and abandoned unexplodedordnance in soil. An approach to the problem that has met with somenoted success, and that holds the promise of future improvement,involves chemical sampling of gaseous emanations from explosivematerials such as TNT in soil. These emanations may include molecules ofthe actual hidden explosive substances that penetrate and partition intosoil gases or they may include other identifier or marker molecules thatcan be linked to the presence of explosives in the soil. Examples ofsuch identifier or marker molecules may include chemical breakdownproducts or manufacturing impurities of the explosive materials ofconcern.

Challenges exist relating to detection and measurement of targetanalytes in this context. These challenges stem primarily from theextremely low concentration of explosive molecules or other markersubstances typically present in soil gas. Sometimes such concentrationsare below the level of parts per billion. Low concentrations can resultin problems of long sampling times necessary to collect enough targetanalyte for accurate detection and/or quantitation. Therefore,strategies are needed for optimizing the collection of the explosivecomponents or other target analytes so that sensitivity of detection ismaximized and sampling time is minimized.

For purposes of this disclosure, detection of explosive substances isbut one embodiment wherein the principles of the invention can besuccessfully applied. The methods and apparatuses described and claimedherein can be adapted and applied beneficially to a broad range ofchemical sampling challenges wherein low concentration of target analytemakes accurate detection and quantitation difficult.

Solid-phase microextraction (SPME) techniques have been the subject ofconsiderable study in recent years, and SPME is emerging as a favoredmethod for sampling of low concentration explosives and other analytes.References describing SPME techniques, specifically as regards toexplosives detection include “Trace Analysis of Explosives in SeawaterUsing Solid-Phase Microextraction and Gas Chromatography/Ion Trap MassSpectrometry”, S. A. Barshick and W. H. Griest, Anal. Chem. 1998, 70,3015-3020; “Trace Explosives Signatures from World War II UnexplodedUndersea Ordnance”, M. R. Darrach, A. Chujian, and G. A. Plett, Environ.Sci, Technol. 1998, 32, 1354-1358; “Application of Solid-PhaseMicroextraction to the Recovery of Organic Explosives”, K. P. Kirkbride,G. Klass and P. E. Pigou, J Forensic Sci., 1998, 43(1), 76-81. Each ofthe references cited above describes generally the use of SPME fibers.Typically, such fibers are fine (˜0.25 mm OD) silica fibers coated witha thin layer of a sorbing material. SPME fibers are often coated with asorbent chosen or engineered to have a high propensity to sorb certainanalytes of interest. The fibers are exposed to a gaseous or liquidenvironment from which a target analyte sample is to be extracted. Ingeneral, low (near ambient) temperatures are required for optimalsorption of explosive gases from air.

After a sample is collected, the fiber can then be conveniently insertedinto a gas chromatograph (GC) by placing the fiber into the inlet of aGC apparatus. One common way to accomplish this is to use a needle topuncture a septum covering the GC inlet, and a syringe plunger to pushthe fiber (containing sorbed analytes) through the needle into the GCapparatus. Next, the fiber is rapidly heated to drive off the analytessorbed to the sorbent substance coating the fiber. The analytes are thenswept into the GC column for normal separation and quantitation.

Typically, SPME sampling involves placing the SPME fiber in theheadspace above a contaminated or potentially contaminated test subjectmaterial (for example, soil). Analytes then passively diffuse throughthe headspace and some ultimately adhere to the fiber. For gaseoussamples of low concentration (such as in the case with explosives insoil gases), diffusion of the analytes through the gas to the SPME fibercan be a rate limiting step, resulting in long sampling times. This isespecially true for instances wherein it is necessary for equilibrium tobe reached, as is the case, frequently, in quantitation studies.“Solid-Phase Microextraction”, Z. Zhang, M. J. Yang and J. Pawliszyn,AnaL Chem. 1994, 66(17), 844A-853A; “Headspace Solid-PhaseMicroextraction”, Z. Zhang and J. Pawliszyn, Anal. Chem. 1993, 65,1843-1853.

SPME has been shown to successfully collect target analytes in lowconcentration in gases and liquids. An opportunity, however, exists foroptimization of SPME techniques, and further, a need remains for anoptimized method and apparatus for extracting target analyte substancesfrom volumes of gases containing those substances in low concentration.The need is especially apparent as regards to overcoming problemsassociated with slow equilibration and long sample times.

It is noted that the assignee of this application, at the time thepresent application is made, also has a separate patent application(09/205,158, Chambers, et al.) pending before the USPTO pertaining to adifferent use of chemical sorption in the context of detecting buriedmunitions. It is submitted, however, that the technology described andclaimed in that application is distinct from the novel SPME techniquesand apparatuses of the present disclosure, both in terms of theoreticalprinciples and application.

SUMMARY OF THE INVENTION

The present invention provides a method and apparatus for samplingtarget analytes in which the target analytes, themselves, are in vaporphase or are suspended in a gas, as in the case of emanations fromexplosive materials present in soil. In one aspect, the invention uses anovel technique of solid-phase microextraction wherein traditional SPMEfibers are omitted in favor of using a new SPME capillary. Thistechnique is augmented in one described embodiment by using heatingmeans (for example, microwave heating) to increase gas partitioningwhere analyte may be present either in gas and liquid, gas and solid, orgas, liquid and solid components present within the matrix to beanalyzed. In another aspect, the invention utilizes the heating (such asmicrowave heating) to increase gas partitioning to enhance samplecollection even where traditional SPME fibers are used.

Advantages and novel features will become apparent to those skilled inthe art upon examination of the following description or may be learnedby practice of the invention. The objects and advantages of theinvention may be realized and attained by means of the instrumentalitiesand combinations particularly pointed out in the appended claims.

DESCRIPTION OF THE FIGURES

The accompanying drawing, which is incorporated into and forms part ofthe specification, illustrates embodiments of the invention and,together with the description, serves to explain the principles of theinvention.

FIG. 1 is a schematic illustration of an apparatus for detecting gaseousemanations from a land mine buried in soil. Included in the figure areFIG. 1A and FIG. 1B, expanded detail cross section depictionsillustrating two approaches to employing SPME in the context of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a method and apparatus for improvedchemical detection of target substances present in a gas. In one aspect,the invention utilizes heating, including, for example, microwaveheating, to increase partitioning of target analyte so as to favorgaseous phase. In another aspect, the invention employs solid-phasemicroextraction using sorbent material either in a traditional passive(headspace) context, or alternatively, in an optimized, non-passivetechnique involving the channeling of gases. This channeling involvesmoving gases potentially containing one or more target substancesthrough a tube, such as a capillary tube, in order to carry those gasesinto close proximity to the sorbent material. In this way, thelikelihood of capturing the target chemical(s) in a short exposure timeis improved versus, for example, passive headspace sampling. Twospecific embodiments are described in connection with this channeling.In the first, a traditional SPME fiber coated with sorbent is used andsupported within a capillary tube through which gases are channeled. Inthe second, the SPME fiber is omitted and replaced by a tube, such as acapillary tube, with SPME sorbent material coating the interior walls ofthe tube. In either of these two embodiments, the gas potentiallycontaining target chemical(s) passes in close proximity to the sorbentmaterial, thus improving the likelihood of capture of the targetchemical(s) in a short exposure time.

The example used throughout this disclosure is that of detectingexplosives in soil. A typical soil comprises a solid phase, a liquidphase (usually water) and a gas phase (usually air). Under ambientconditions, most explosives present in soil reside in a solid or liquidphase, with only a small portion partitioning into the gas phase. Thislow propensity to partition from the water into the air results in anextremely low Henry's law constant. The water-air partitioning is anexponential function of absolute temperature, so that by raising thetemperature from ambient temperature, for example, from 25° C. to 70°C., gas partitioning increases by at least two orders of magnitude.Increasing the soil temperature by even 10° C. (above ambient) will havea favorable effect which results in availability of more targetsubstance in the gas phase, thus increasing the likelihood of detection.Improved detection results from increasing amount of target substance inthe gases to be sampled and improving the thoroughness of extraction oftarget substance from the carrier gas. Both of these independentparameters are addressed by the present invention.

Concerning temperature, optimized sampling can be obtained by heatingsoil while keeping the SPME sorbent cool. This is mentioned in the 1994Zhang, et al., Anal. Chem. reference noted above. One aspect of thepresent invention is to accomplish heating of soil, in one embodiment,using microwaves. FIG. 1 illustrates an apparatus configurationaccording to an embodiment of the invention. The figure shows howmicrowave horns may be used in conjunction with two alternative SPMEconfigurations to achieve the desired soil heating. The figureillustrates a landmine 5 buried in soil 10 with a plume 15, for example,containing TNT molecules. As described previously, molecules such asthose in the plume 15 are likely to be present in a combination of solidand liquid phase, with only a small concentration partitioned into thegaseous (air) component of the soil in which the plume resides. SinceSPME sampling according to this embodiment depends on capturing targetmolecules diffused into and present in gaseous soil component, it isdesirable to heat the soil in order to drive more target molecules intothat phase. Optimal heating temperatures will depend on a givenapplication and the target molecules sought to be detected, however, forexample, partitioning of molecules emanating from a plume containing TNTcan be successfully enhanced by heating soil. Optimally, the targettemperature is in the range between 80° C. and 110° C. Heating the soilto too high a temperature (or producing hot surfaces that can contactthe vapor sample) may cause target chemicals to decompose or otherwisedegrade thereby rendering them more difficult to detect. Insufficientheating may result in inadequate partitioning of target chemicals intothe gas phase.

As illustrated in the figure, an enclosure 20 (shown with the sidefacing the reader cut away) is provided. The enclosure 20 can be in theform of a box positioned atop the surface 11 of the soil 10. At leastone heating element, which in the illustrated embodiment is at least onemicrowave-generating element, is provided. The at least one heatingelement is capable of directing microwave radiation to and through aportion of the soil 10 beneath the enclosure 20. In the exampleillustrated, the at least one microwave-generating element actuallycomprises two microwave horns 25 directed downward toward the soil 10.It is acknowledged, however, that the appropriate number andconfiguration of such horns used for a given application will depend ona number of variables such as soil condition, characteristics of thetarget chemical substance(s) and features of the plume. Generally,though, two microwave horns will provide the desired increase inpartitioning. It is also acknowledged that while microwave heating isused in the illustrated embodiment, other heating means may satisfy theobjectives of the invention equally well. Advantages are apparent,though, where the heating element heats the target soil, for example,without at the same time directly heating the air surrounding theheating element or producing any surfaces hot enough to cause targetchemicals to degrade. Microwave heaters or other forms of radio waveheaters are well suited for the purposes of the invention. It is noted,and discussed further below, that it is beneficial to avoid heating thesorptive surface that will be used to collect the sample. Radiantheaters, for example, may satisfactorily heat the soil causingpartitioning of target chemical into gas phase. At the same time,however, they may also heat the air inside the enclosure 20 as well asthe sorptive surface intended to capture target chemicals, resulting ina decrease in sample collection efficiency. In fact, it is sometimesbeneficial to cool the sample-collecting element (referred to herein asa SPME assembly 30), as will be described.

Also, as illustrated in the figure, a SPME assembly 30 is provided inassociation with the enclosure 20. The SPME assembly 30 includes SPMEsorbent positioned on a substrate according to at least two possibleconfigurations to be described, shortly, in reference to FIGS. 1A and1B. According to at least one embodiment, the SPME assembly 30 is inoperative association with a device (not shown) for actively channeling(moving) air from within the enclosure to a region outside of theenclosure in a fashion that air passes in proximity to the SPME sorbent.The bold curved arrow in the figure depicts one possible manner in whichsoil gas can exit the enclosure, after passing by the SPME assembly 30,in those instances wherein gases are actively channeled. Suitable SPMEsorbent materials for specific applications are known to persons skilledin the art of solid-phase microextraction, and examples of SPMEcompounds are described in various catalogs and other publications,including the SUPELCO™ Chromatography Products 1996 catalog (publishedby Supelco, Inc., Supelco Park, Bellefonte, Pa.), which is incorporatedherein in its entirety.

In operation, for purposes of the illustrated embodiment, the microwavehorns 25 shown in the figure direct microwave-radiation generallydownward into the soil 10. In so directing radiation, molecules oftarget chemicals present in liquid or solid phase within the plume 15heat up and are driven into the gas phase, thereby increasing theconcentration of target analyte chemical(s) in the soil gas beneath theenclosure 20. The soil gas will tend to move through the soil matrix,and a portion of the soil gas will rise with some of the soil gasentering the enclosure 20. This effect of the soil gas entering theenclosure 20 is enhanced, by some degree, as a result of the heating andalso by movement of gases out of the enclosure as depicted by the boldcurved arrow in the figure.

Two different SPME assembly configurations are illustrated in FIGS. 1Aand 1B. The configuration shown in FIG. 1A involves using a typical SPMEfiber in the inventive SPME assembly 30. Two different applications forthe configuration in FIG. 1A will be described, one involving passivesample collection and the other involving active channeling of gases forenhanced sample collection. The configuration in FIG. 1B shows a SPMEassembly 30 wherein SPME sorbent is supported on the inside walls of atube rather than on a fiber. As will be described, this configurationhas special applicability where active channeling of gases is used.

FIG. 1A shows a cross section of a fiber embodiment of the SPME assembly30. The FIG. illustrates a central fiber 60 coated with sorbent 50. (Forpurposes of this disclosure including the figures, the relativethicknesses of sorbent 50 and fiber 60 are not shown to scale.Commercially available SPME fibers, exhibiting the thickness of sorbentmaterial typically inherent in commercial SPME fibers, will adequatelyserve the purposes of this invention.) In the illustration, a support40, such as a syringe plunger, holds the fiber 60. (As noted previously,a syringe can be used advantageously for convenience in later insertionof the fiber into an analysis instrument, such as a GC.)

Also shown in FIG. 1A is an insulator 35. In the illustrated embodiment,the insulator 35 is a tube, open at the bottom, surrounding the fiber 60supporting the sorbent 50. The function of the insulator 35 is toprevent the SPME fiber from heating significantly (or at all) as aconsequence of the operation of the heater used to elevate thetemperature within the soil. (In the illustrated case microwave horns 25are used, in which instance significant direct heating of air inside theenclosure 20 is largely averted. However, insulation and/or cooling ofthe SPME element 30 can be advantageous even where microwave or otherradio wave heating means is employed. Certainly, advantage is obtainedin using an insulator where other forms of heater, for example a radiantheating apparatus, are used which may tend to heat the air inside theenclosure.) The sorbent collects sample most effectively when maintainedat ambient temperature, or even below, depending on the sorbent used andthe circumstances of sampling. Where cooling is desired, the insulatortube 35 can be replaced with another form of tube or equivalentenclosing structure capable of being actively cooled, such as byelectrical or other means known in the cooling art. Suitable coolers caninclude (but are not limited to): Peltier cooling, gaseous cooling suchas the CO₂ method described in U.S. Pat. No. 5,496,741 (Pawliszyn, J.B.) and other circulating liquid or gas coolers.

The configuration in FIG. 1A can serve either to optimize passiveheadspace sampling, or alternatively, to benefit from active channelingof gases. Even where no effort is made to actively move gases past theSPME fiber, improved sample collection is obtained over previous art asa result of the heating of the soil (to increase partitioning) and usingthe enclosure 20 (to provide a degree of concentration). Heating, asillustrated, may be accomplished through use of the microwave horns 25and the enclosure 20.

FIG. 1A also shows an example of how active channeling of the gases pastthe SPME fiber could take place consistent with the principles of theinvention, thereby increasing the likelihood of capture of targetsubstances by the SPME sorbent. In the illustrated case, gases may becaused to flow past the SPME fiber, for example, by being drawn throughthe insulator tube 35 in the direction shown by the finely printedarrows. In the illustrated example, gases are allowed to gases areallowed to exhaust through an opening in the tube, however, any exhaustmeans enabling the flow of gases past the SPME fiber would satisfy theends of the invention.

As discussed, in operation the apparatus may also include a device formoving air from within the enclosure 20 so that it passes in proximityto the SPME fiber and then out of the enclosure 20 as illustrated by thebold arrow in FIG. 1. The air-moving device, not shown in the figure,can include any of the various pumps, fans and other air moving devicesthat are well known in the art. It is possible for the invention tooperate successfully without such an air moving device, with gasessimply diffusing throughout the enclosure 20, including in proximity tothe SPME fiber, however, as noted above, passive collection can be veryslow. Accordingly, for purposes of the present invention, it may bedesirable to actively move gases toward and in proximity to the SPMEfiber. Also shown in the Figure is the ceiling 21 of the enclosure toclearly illustrate that the tube 35 penetrates the enclosure and permitspassage of gases from within the enclosure 20 to a region outside of theenclosure.

By actively moving gases from the enclosure, through the tube, and inproximity to the fiber, diffusion distances are minimized and thelikelihood of target analytes being captured by the sorbent material 50on the fiber is increased.

FIG. 1B shows different embodiment employing a novel SPME configurationthat does not use the traditional SPME fiber. In this instance, ratherthan a fiber, a SPME capillary is provided. The SPME capillary comprisesa tube, such as a capillary tube 70, including two open ends 71 and 72,and a central channel 73. Sorbent 50 coats the inside of the capillarytube 70 and, specifically, the surface bounding the channel 73, asdepicted in the figure. (As in the previously described embodiment, therelative thicknesses of the sorbent 50 as compared with the otherillustrated elements including the walls of the tube 70 are not shown toactual scale. Appropriate actual thicknesses of sorbent material willdepend on the particular sorbent material deployed, however, suchthicknesses will be similar to those used for commercial SMPE fiberswith similar sorbents.) In this instance, also, an air-moving device isnecessary in order for the capillary tube embodiment to functionoptimally. As with the previously described embodiment, various airmoving devices including suction, fans and pumps are well known andcould be successfully adapted to satisfy purposes of the presentinvention.

FIG. 1B shows a support member 40 supporting the SPME capillary(comprising the capillary tube 70 and the sorbent 50). Functional tubes70 include tubes ranging in size from 0.75 mm to 7.5 mm ID, however,depending on a particular application, larger or smaller tubes may besuitable. FIG. 1B also illustrates a conduit 45 is depicted within thesupport member 40. As noted, the capillary tube 70 includes two openends. The first open end 71 is open to the interior of the enclosure 20and the second open end 72 adjoins the support member 40 so that thecentral channel 73 of the capillary tube 70 substantially aligns withthe conduit 45 of the support member 40.

During collection of sample using the configuration illustrated in FIG.1B, air movement means is applied so that gases (including carrier gaswith perhaps entrained target analytes) are drawn into and through thecapillary tube 70. In particular, gases pass via the first open end 71of the capillary tube 70, through the capillary tube central channel 73,and thereby past the sorbent 50 lining the interior surface of thecapillary tube 70. The gases then pass out of the capillary tube centralchannel 73, through the second open end 72 of the capillary tube 70 andthen into the conduit 45 (or other equivalent structure through whichgases can pass). The finely printed arrows in FIG. 1B illustrate thedirection of channeled flow of gases in this embodiment. As a result ofthe channeled flow of gases just described, recovery of analytes fromcarrier gas is enhanced versus a technique that employs passivediffusion. Likewise, the time for equilibration is minimized since thegases (for example, soil gas) are drawn in close proximity to the SPMEsorbent 50. The distance of gas phase diffusion is reduced as comparedwith traditional headspace SPME techniques using traditional SPMEfibers. Again, as before, also shown in the Figure is the ceiling 21 ofthe enclosure to clearly illustrate that the conduit 45 penetrates theenclosure and permits passage of gases from within the enclosure 20 to aregion outside of the enclosure.

It is noted that although the desorption approach described fortraditional SPME fibers, whereby fibers can be injected into a GC inlet,has been standardized, desorption of target chemicals from the inventiveSPME capillary may require additional steps. These include using knownand commonly used thermal desorption techniques combined with, forexample, causing a purge flow of gas through the SPME capillary in orderto carry desorbed target chemicals into the GC inlet.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the appended claims. It is intendedthat the scope of the invention be defined by the claims appendedhereto. The entire disclosures of all references, applications, patentsand publications cited above are hereby incorporated by reference.

We claim:
 1. An apparatus for capturing chemical substances in a carriergas comprising: a tube including an inner surface, first and second openends, and a channel therebetween, the channel bound by the inner surfaceof the tube, a solid-phase microextraction fiber suspended within thechannel of the tube, temperature regulation means selected from thegroup consisting of thermal insulation at least partially enclosing thetube and cooling means in operative association with the tube which,when activated, causes the solid phase microextraction fiber to attain atemperature lower than that of ambient air outside of the tube, andgas-moving means for moving the carrier gas through the channel.
 2. Theapparatus of claim 7 further comprising a partially enclosed structureincluding: an interior region, support holding the tube so that when thegas moving means is actuated, gases move from the interior region of thepartially enclosed structure, through the channel of the tube and awayfrom the partially enclosed structure, and an opening in the partiallyenclosed structure adapted to communicate with a soil region having asurface so that gases emanating from the surface of the soil region canpass into the interior region of the partially enclosed structure. 3.The apparatus of claim 2 further comprising at least one heater.
 4. Theapparatus of claim 3 wherein the at least one heater is selected fromthe group consisting of microwave heaters and radio wave heaters.
 5. Anapparatus for capturing chemical substances in a carrier gas comprising:a) a tube including an inner surface, first and second open ends, and achannel therebetween, the channel bound by the inner surface of thetube, b) a solid phase microextraction fiber suspended within thechannel of the tube, c) gas-moving means for moving the carrier gasthrough the channel, d) a partially enclosed structure including aninterior region, support holding the tube so that when the gas movingmeans in actuated, gases move from the interior region of the partiallyenclosed structure, through the channel of the tube and away from thepartially enclosed structure, and an opening in the partially enclosedstructure adapted to communicate with a soil region having a surface sothat gases emanating from the surface of the soil region can pass intothe interior region of the partially enclosed structure, e) at least oneheater selected from the group consisting of microwave heaters and radiowave heaters, and f) cooling means in operative association with thetube whereby, when activated, the cooling means causes the solid phasemicroextraction fiber to attain a temperature lower than that of ambientair outside of the tube but otherwise within the interior region of thepartially enclosed structure.